This invention relates generally to injection molding machines and, more particularly, to injection molding machines including means for selectively opening and closing the hot sprue of a machine molding assembly designed for molding centrally apertured record discs, such as video information discs.
An example of an injection molding machine of this particular type is disclosed in copending and commonly assigned patent applications U.S. Ser. No. 847,367, filed in the name of J. R. Holmes et al., now abandoned, and entitled "METHOD AND MEANS FOR REPLICATING CENTRALLY APERTURED VIDEO DISC RECORDS," and U.S. Ser. No. 031,205, filed in the name of J. R. Holmes, now abandoned, and entitled "MOLDING APPARATUS FOR PRODUCING CENTRALLY APERTURED RECORD DISCS." The apparatus described in these applications comprises an injection molding machine having a molding assembly with first and second mold halves reciprocally movable between a closed position, wherein molten disc-forming material of plastic or the like is injected into an annular mold cavity to form a video information disc, and an open position wherein the mold parting line is opened and the resultant molded video information disc is removed from the machine. The annular mold cavity is defined by a pair of planar, disc-shaped stamping dies carried by platens which are in turn reciprocally movable toward and away from each other along with the respective mold halves.
In injection molding machines of this general type, it is well known to inject molten plastic material under pressure through an injection sprue bushing and into the mold cavity defined by the two disc-shaped stamping dies. When the mold cavity is filled with the molten plastic material, the surrounding machine structure including the respective mold halves and the sprue bushing are cooled by a circulating liquid coolant to reduce the temperature of the molten plastic material and thereby solidify the material. In this manner, the molded record disc is quickly solidified within the mold cavity whereupon the solidified disc can be removed from the machine and the cycle restarted, thereby allowing a rapid production rate of the record discs. However, a substantial amount of plastic material within the sprue bushing has a relatively large thickness compared with the plastic material within the mold cavity, whereby the plastic material within the sprue bushing takes the longest time to solidify. The cycle time for molding the record discs is therefore dependent upon the cooling time of the sprue material, and not upon the cooling time of the disc per se. Moreover, this sprue material when solidified comprises wasted plastic material which must be removed from the molded record disc, as by means of a punch assembly or the like, to yield the desired thin, substantially planar record disc including the required central aperture.
Formation of the central aperture by means of a conventional punch assembly of the type disclosed in the above-referenced copending applications poses a variety of problems in the production of the record discs. For example, the punch step requires a certain cycle time to perform, thereby prolonging the cycle time for each record disc. Moreover, the use of a punch assembly to form the apertures after solidification of the record discs results in aperture shapes having a concentricity dependent upon the state of wear of the punch assembly. As punch assembly wear progresses, the likelihood of the creation of debris in the vicinity of the aperture increases, and such debris can deleteriously affect the quality of subsequently molded record discs. Finally, the use of a punch assembly occasionally results in cracking of the discs in the region of the aperture, whereby the discs must be rejected as scrap.
Some attempts have been proposed to mold an aperture in the record disc prior to solidification of the disc-forming material, and thereby avoid use of a punch assembly. See, for example, copending U.S. application Ser. No. 202,824, Oct. 31, 1980, filed in the name of A. Cane et al. and entitled "HOT SPRUE VALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE." However, the aperture-molding valve assembly of this application still requires a solidified portion of the disc-forming material with the region of the formed aperture to be removed from the machine as wasted scrap material.
The present invention provides an injection molding machine including an improved mold assembly particularly designed for use in making relatively thin, substantially planar record discs, such as video information discs, wherein the disc production rate is independent of the solidification time of molten plastic material in the sprue bushing region of the injection machine. The present invention fulfills this need by providing a valve assembly for isolating from the mold cavity the molten plastic material within the sprue bushing during solidification of the plastic material in the mold cavity while at the same time eliminating waste of any plastic material within the aperture region of the disc.